Kelemvor: OK, I am now about to judge your soul.
Adventurers: yep, we're ready for this formality on our way to the upper planes.
Kelemvor: I'd send you to Arvendor, because up to 9th level, you fought against the slavers of Faerun, first gangs, the Lolthite, then Thayvians...
Adventurers: we're the good ones!
Kelemvor: However, starting at level 10, you got your stronghold and... set up slaver raids?
Adventurers: ah, this time, yes, but we had no choice: do you know the cost of unskilled labor these days???
Going with the daily rates of some specialists is difficult because honestly, it's difficult to estimate how much work can a fantasy laborer do in a day for a given speciality. A better approach might be to derive it from global cost of a "finished" product.
A stronghold is one of the few money drains in 5e, if you don't allow magical items shopping, so don't be afraid of jacking up the cost if they intend to care a lot about their manor. If you want an idea of the duration of the total cost, here's what Wikipedia has on the Chambord Castle, which may be bigger than what your adventurers need (but for which we have accounting data):
Regardless of who designed the château, on 6 September 1519 Francis Pombriant was ordered to begin construction of the Château de Chambord.
[17] The work was interrupted by the
Italian War of 1521–1526, and work was slowed by dwindling royal funds
[18] and difficulties in laying the structure's foundations. By 1524, the walls were barely above ground level.
[17] Building resumed in September 1526, at which point 1,800 workers were employed building the château. At the time of the death of King Francis I in 1547, the work had cost 444,070
livres.
[18]
Unskilled labor cost varied, but the order of magnitude at this period was around 0,1 to 0,15 livre a day -- it's difficult to compare costs, but going by a day's wage to calculate costs might be sufficent for rpgs -- and if we go with 2sp a day as per the DMG, the total cost was (444,070/0.1)x2sp, or 600,000 to 900,000. The castle is 120mx160m. The cost however can be significantly brought down by magic.